Use src/lib/w32/w32io.c for client

Replaces w32_writev_socket() and w32_readv_fd().  Split w32types.h off
w32misc.h, to avoid putting irrelevant stuff into client tarball.

src/client/ringbuf.c

@@ -36,16 +36,7 @@
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>
-#ifndef _WIN32
 #include <sys/uio.h>
-#include <unistd.h>
-#else
-#define readv(fd, iov, iovcnt) \
-    w32_readv_fd((fd), (iov), (iovcnt))
-#define writev(fd, iov, iovcnt) \
-    w32_writev_socket((fd), (iov), (iovcnt))
-#endif
-#include "misc.h"
 #include "ringbuf.h"
 
 /*